CdTe heterostructures

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Journal of Crystal Growth 79 (1986) 734-740 North-Holland, Amsterdam

PREPARATION AND CHARACTERIZATION OF C d S / C d T e HETEROSTRUCTURES A.M. MANCINI and L. VASANELLI Dipartimento di Fisica dell'UniversitY, Unit~ GNSM/CISM, Via Amendola 173, 1-70126 Bari, Italy

and C. DE BLASI Dipartimento di Fisica dell'UniversitY, Unit~ GNSM/CISM, Lecce, Italy

The deposition of cadmium sulphide films on cadmium telluride substrates, by closed-tube chemical vapour deposition, has been investigated using hydrogen as a transporting agent. Several deposition runs were performed in order to determine the values of the experimental parameters necessary to produce the best quality films. These were obtained at a source temperature of 730°C and deposition temperature of 630°C, with a hydrogen pressure of 250 Torr. Films grown under these conditions generally have a good epitaxial relationship. For slightly different values of the growth parameters, the films contain pinholes and hollow crystalline grains, as is often the case for other II-VI compounds grown by the vapour phase.

1. Introduction Cadmium sulphide thin films have been extensively studied for their applications in heterojunction solar cells [1] and optoacoustic transducers [2]. Many studies have been reported concerning the growth and the morphology of CdS films, deposited on different substrates (Ge, GaAs, SrF2, CdTe, etc.) [3-8]. One of the most interesting substrates is CdTe, because it has been demonstrated that CdS/CdTe heterojunctions can be prepared with photovoltaic conversion efficiencies greater than 10% [9]. Several deposition methods have been used to grow CdS films on CdTe substrates: evaporation [10], spray-pyrolysis [11], close-spaced vapour transport [12], hot-wall epitaxy [13], open-tube vapour transport [9] and electrodeposition [14]. In this paper we report results concerning the epitaxial growth of CdS thin films on CdTe single crystals by closed-tube chemical vapour deposition (CT-CVD) method. This method has been successfully used in the past to grow CdS single crystals and thin films [15], in addition to other II-VI thin films such as CdTe [16] and ZnTe [17]. However, to our knowledge, no results have been

reported on the growth of CdS films on CdTe crystals. The experimental procedure to grow the CdS films is described and their morphological properties are investigated and correlated to the growth parameters, in order to determine the best growth conditions. The growth mechanism is also discussed.

2. Experimental CdS films were grown by using the closed-tube chemical vapour deposition technique. The transport of CdS from the source to the deposition region is performed by the reaction with hydrogen according to the equation: CdS(s) + H2(g ) = H2S(g ) + Cd(g),

(1)

where (s) and (g) designate solid and gaseous species, respectively. The experimental apparatus is reported elsewhere [18]. The growth was performed in a quartz ampoule of suitable shape, into which about 1 g of CdS powder (5N purity from Cerac Ltd.) was introduced. The substrates were (111) CdTe single crystals, grown by Bridgman method. The slices,

0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

A.M. Mancini et aL / Preparation and characterization of CdS/ CdTe heterostructures

with typical dimensions of 0.4 × 0.4 × 0.1 cm 3, were cut from a crystalline ingot and then mechanically lapped and polished, to a mirror finish, by diamond pastes. A chemical etch with H202: H 2 0 : H F ( 2 : 1 : 3 ) was used in order to identify the Cd face [19]. Finally the samples were etched in a HC1 solution, carefully rinsed in doublydistilled water and blown dry with nitrogen, before being introduced into the deposition side of the growth ampoule, with the Cd face up. After evacuating the growth vessel using an ion pump, H 2 was introduced to a pressure of about 250 Torr at room temperature. The growth ampoule was then sealed off and placed in a two zone furnace. Before imposing the temperature profile to grow the CdS film, a thermal annealing of the substrate was carried out at a temperature of about 800 o C for 30 min. The morphology of the CdS films was studied by means of an optical microscope and a scanning electron microscope (SEM). The growth rate was determined by evaluating the mean thickness of the film by cross section observations. In some cases energy dispersive X-ray (EDX) microanalysis measurements were carried out, in order to check the presence of a mixed layer at the interface between the CdS film and the CdTe substrate. X-ray diffraction measurements were also carried out on some samples using Cu K a radiation.

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500

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Fig. 1. E x p e r i m e n t a l d e p o s i t i o n rates versus Ta for C d S films g r o w n with T~ = 800 o C.

the films is concerned, the more interesting results were obtained when the temperature gradient, AT = T~- Td, was between 40 and 120°C. In fact, in this case the films show a more uniform and homogeneous structure. Further deposition runs were carried out by fixing Td and varying T~, so that their difference was in the before mentioned range. Fig. 2 shows the deposition rate as a function of AT, as measured on films grown at Td = 630°C and Td = 710°C, respectively. In this case, the deposition rate was again about 4-5 /~m/h and quite independent of the temperature gradient whilst in contrast the morphology of the film strongly

3. R e s u l t s and d i s c u s s i o n

Several deposition runs were carried out in order to determine the experimental conditions necessary to grow the best quality films. Some preliminary depositions were performed by fixing the source temperature, T~, at about 800°C, and varying the deposition temperature, Td, in the range 500-760°C. Fig. 1 shows the deposition rate as a function of T a. Its value is almost independent of Td over wide range (540-720°C) and is about 4 - 5 /~m/h. It is interesting to observe that this value is higher than those obtained for the growth of CdTe [16] and ZnTe films [17] by the same deposition technique but using different transporting agents. As far as the morphology of

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Fig. 2. E x p e r i m e n t a l d e p o s i t i o n rates versus A T for CdS films grown w i t h Td = 7 1 0 ° C (O) a n d Td = 6 3 0 ° C (A).

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A.M. Mancini et al. / Preparation and characterization of CdS/CdTe heterostructures

depended on the value of AT. Fig. 3 shows two examples of films grown with Td = 710°C and AT equal to 100 and 120°C (figs. 3a and 3b respectively), and Td = 630°C and AT equal to 70 and 100 °C (figs. 3c and 3d, respectively). The hexagonal shape of the growth patterns of the sample shown in fig. 3d indicates that at Td = 630°C and A T = 100°C the best epitaxial relationship of the film is obtained. When the temperature of the substrate is raised or AT is slightly changed, the growth pattern changes toward a circular shape. Films grown at lower or higher AT always have a worse crystalline quality. These results are in good agreement with those reported in ref. [15] for the growth of CdS films on (111) Ge substrates. Figs. 4a and 4b show micrographs of the surface and the cross-section of the sample of fig. 3d, respectively. The film appears uniform and homogeneous and the surface shows very few defects. Fig.

5 shows a magnification of a typical defect. It is a hole of triangular shape, in which the growth initially occurred in a disordered way, probably because of the presence of some imperfection of the substrate. Films which have a poor epitaxial relationship generally also show a higher density of surface defects and holes. X-ray diffraction measurements show that the films have a hexagonal wurtzite structure. This result is in good agreement with the general features of the CdS films, which grow in this stable hexagonal structure when deposited on a (111) A-face of a cubic crystal [4]. EDX microanalysis measurements showed no evidence of a mixed layer between the CdS film and the CdTe substrate but did reveal the presence of a small A1 contamination in the film, probably due to the growth vessel. The morphology of the CdS films indicates that

Fig. 3. Optical micrographs of CdS films grown on the Cd face of (lll)CdTe. The growth conditions are Ta = 710 o C, AT = 100 ° C (a), A T = 1 2 0 ° C (b) and Ta = 630°C, A T = 7 0 ° C (c), A T = 1 0 0 ° C (d). Magnification 135 × .

A.M. Mancini et al. / Preparation and characterization of C d S / CdTe heterostructures

737

Fig. 4. Scanning electron microscopy photographs of a CdS film surface (a) a n d cross-sectional view (b). The sample is the same of fig. 3d.

the growth of the crystalline grains occur mainly along the direction of the c-axis. Such a behaviour is well evidenced in fig. 6 which shows a sample grown a t T d -= 710°C and A T = 40°C, in which

the layer exhibits well developed hexagonal prisms. In this case the low value of AT gives rise to a low flux of vapour, which does not allow an uniform growth of the film. Fig. 7 shows the inner part of a

Fig. 5. SEM photograph of a typical surface defect.

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A.M. Mancini et al. / Preparation and characterization of CdS/CdTe heterostructures

Fig. 6. SEM photograph of CdS film surface with several prisms developed along c-axis.

fracture at the surface of a sample grown at Td = 710°C and AT----- 100°C. In this case it is evident that the film consists of co-oriented prisms. This

growth mechanism is quite similar to those observed for CdS single crystals [20], in agreement with Bliznakov [21] who shows that the growth

Fig. 7. SEM photograph showing a surface fracture with co-oriented prisms.

A.M. Mancini et al. / Preparation and characterization of C d S / CdTe heterostructures

739

fig. 8, where their presence on the top faces of some prisms is evident. The sample of fig. 8 was grown using optimized growth parameters. However, when the growth conditions are far from the best values, the presence of these defects is enhanced, as shown by the micrograph of fig. 9, in which several hollow prisms, with different orientations, are evident.

4. Conclusion

Fig. 8. SEM photograph of some prisms with several pinholes and cavities. mechanism of the perfect crystals also applies to epitaxial growth on foreign substrates. The growth in the c-direction is dominant in the initial growth stage, while the growth in the lateral direction becomes predominant in the final stage. Such a growth mechanism can also give rise to the presence of pinholes at the surface of the crystal or, in some cases, t o hollo w crystals. In fact, they have been found in most I I - V I compounds with a wurtzite structure, for example CdS, CdSe [20], Z n O [22] and ZnS [23]. In our films we also observe pinholes, as shown in the micrograph of

In conclusion, good quality epitaxial CdS films can be grown on (111) CdTe crystals b y CT-CVD method by a suitable choice of the growth parameters. Their values are: source temperature 730°C; deposition temperature 630°C; hydrogen pressure 250 Torr. Under these conditions the growth rate is about 5 / ~ m / h . The growth of CdS films occurs by the same growth mechanism as CdS single crystals from the vapour phase.

Acknowledgments The authors are strongly indebted to Professor A. Rizzo for very stimulating discussions, to Dr. D. Ferro for some electron microscopy observations and to Dr. A. Quirini for his valuable collaboration. The technical assistance of Mr. G. Casamassima is also acknowledged. The work has been partially supported by the Ministry of Education of Italy and Progetto Finalizzato "Chimica Fine e Secondaria" of CNR.

References

Fig. 9. SEM photograph of disordered growth example, with hollow prisms grown in different directions.

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